Automatic gain control



April 25, 1933. A MATHIEU ET AL 1,905,946

AUTOMAT I 0 GAIN CONTROL Filed April 1, 1932 2 Sheets-Sheet l HFA \FA DLFA INVENTOR GASTON A. MATHIEU ATTORNEY GERAL A ISTED BY W April 1933-G. A. MATHIEU ET AL AUTOMATIC GAIN-CONTROL- Filed April l, 1932 2Sheets-Sheet 2 AQG c I L LUV Patented, Apr. 25, 1933 UNITED STATESPATENT OFFICE GASTON ADELIN MATHIEU, 0] LONDON, AND GERALD ARTHUR ISTED,OF ESSEX, ENG- LAND, assrenons we wear RADIO CORPORATION OF AMERICA, ACORPORATION OF DELA-.

AUTOMATIC GAIN CONTROL Application filed April 1, 1932, Serial No.602,524, and in Great Britain March 28, 1831.

This invention relates to automatic gain control devices for use in highfrequency receivers and more particularly to automatic ain controldevices for use in radio receivers or the purpose of minimizing oreliminating the eifectsof fading.

As is well known one important difiiculty met with in radio receptionand more particularly in radio reception on short waves i. e. on wavesof the order of 80 or meters or less, is that known as fading, and manyattempts have been made to overcome this difiiculty by providing meansfor automatically varying the amplification gain in the receiver withthe intention of causing the sigpal finally indicated, or recorded, tobe of su stantially constant intensity notwithstanding variations in thereceived signal strength obtained upon the receiver aerial.

The principal ob]ect of'the present invention is to provide an improvedautomatic gain control device of simple arrangement and construction,and which shall be capable of varying the gain of a sensitive andpowerful multistage receiver between very wide limits of variations andsubstantially without introducing distortion. Modern highly sensitiveradio receivers are capable of giving very large amplification of theorder of from 50 to 80 decibels when working on a wave length of theorder of 60 to 80 meters, and it will readily be appreciated that it isnot a simple matter to provide automatic means for varying theamplification gain of such a receiver to an extent commensurate with themaximum possible gain thereof.

According to this invention a high frequency r ceiver comprising aplurality of thermionic amplifier members in cascade, is provided withmeans, operated automatically in accordance with the stren th of therectified output from a detector included in said receiver, for varyingthe gain of thermionic amplifier members in said receiver, thearrangement being characterized by the feature that the control operatesfirst to vary the gain of one of the amplifier members, and then, if therectified output still departs from a predetermined value, to vary thegain of a second amplifier member in addition, and

then of a third amplifier member, and so on.

Preferably the control is obtained by utilizing voltage drop set up bythe rectified output of the detector to vary the bias upon the controlelectrodes (control grids in the case of triodes or control grids orscreening rids in the case of screened grid valves) 0 the valves (whichmay include said detector) .Whose gain is to be controlled and therequired successive operation upon the plurality of amplifier members isobtained by providin such normal bias upon the am 11- fier 111cm erswhich are to be controll in succession that a larger variation in gridbias is required substantially to modif the output of a given amplifieryalve, t an is required substantially to modify the output of anamplifier valve whose gain is required to be controlled to compensatefor a lesser degree of fading.

Preferably also the circuit from which the control potentials isobtained, consists of a simple diode circuit in parallel with the normalreceiver detector provided for separating out signal modulation.

Although no limited to its application thereto, the invention isparticularl well adapted for use in connection with highly sensitiveradio receivers of the frequency changing type, and will be describedwith reference to its application to a radio receiver consisting of ahigh frequency am 'lifier, frequency changer, an intermediate requencyamplifier, a second detector and an audio frequency'amplifier. Incertain cases a receiver of this kind may comprise a second frequencychanger and a second intermediate frequency amplifier (operating at adifi'erent.frequency from the first intermediate frequency amplifier)preceding the last detector i. e. that which is provided to separate outthe modulation frequencies.

The invention is illustrated in the accompanying diagrammatic andschematic drawings wherein Fig. 1 schematically shows a superheterodynereceiver embodying the invention,

Fig. 2 shows the essential circuits of the receiver of Fig. 1,

cally a superheterodyne receiver embodying the invention A is areceiving aerial; HFA- a high frequency amlplifier; FC a frequencychanger; LO a ocal oscillator; IFA. an intermedlate frequency amplifier;D a modulation separating detector and LFA a low frequency amplifier,all these parts being of ordinary well known general arrangement andconstruction. AGC is an automatic gain control unit and the leads marked1, 2 3 and 4 are so marked to identify them wit leads shown in Fig. 2which illustrates the circuits of the detector D and automatic gaincontrol unit AGO.

Referring to Fig. 2 the input circuit of the valve V (the valve of thedetector D) is coupled to the preceding IFA stage by a transformer TRThe grid of the valve V is connected through a condenser C to the gridof a diode, or, as shown, a triode V connected as a diode, the cathodeof the valve V being connected'directly to earth at one end and beingearthed at the other through a condenser C The plate of the valve V isconnected to the grid, and, through an indicating instrument M and ahigh frequency blocking impedance R as a choke coil, to a bias batteryas shown; and finally through a second resistance R to earth.

The grid circuits for the audio frequency detector valve V theintermediate frequency amplifier (see Fig. 1), the frequency changervalve (see Fig. 1), and the high frequenc amplifier (see Fig. 1) arecompleted throng leads 4, 3, 2 and 1 respectively to tapping points upona bias battery E,,, the positive terminal of which is connected to oneend of resistance R as illustrated. The impedances R and R may bevariable if desired, and it Will be noted that the latter resistance istapped upon the battery E in order to obtain a required positive voltagefor the valve V This voltage may or may not be necessary according tothe nature and type of valve V The valve V is preferably a socalled'high power detector valve, having a low impedance output and a fairlylow amplification factor, thus permitting a large negative biasing ofits grid, and avoiding all possibilities of grid current rectification.

The valve V can be as shown connected to act as a. diode by connectingthe grid and the plate together, or it could, of course, be replaced bya diode valve. This valve is far from being critical and on manyoccasions it will be found quite suitable to use the grid and filamentof a triode to act as a diode leaving the "plate free. CH and CH arehigh frequency blocking chokes; C is a bypass high frequency condenser,and C C and C are low frequency by-pass condensers. Chokes CH and CH arelow frequency blocking chokes. Transformer TR is the low frequencyoutput transformer for detector valve V and M an indicating instrument.

The position of the tapping points on the battery E are so chosen thatas the received signals vary from very Weak to very strong the reductionof the total gain of the receiver as a whole is obtained first bymodifying only the output of the audio frequency detector, then bymodifying also the out at from the intermediate frequency ampli er,

then by modifying also the output of the' frequency changer valve(modulating valve), and finally, for very strong signals, by modifyingalso the high frequency amplifier so that for the strongest of signalsall the various circuits of the receiver are controlled to reduce theirgain. This effect is obtained by adjusting the constant bias of theaudio frequency detector, so that the said detector operates on a lowportion of the plate characteristic curve, so that a very smallvariation of biasing will have a considerable effect on the detectoroutput.

The constant bias for the other valves to be successively controlled aresuch that said valves operate about mean points on the platecharacteristic curves, which are successively higher, and thereforesuccessively increasing variations of bias are required to producesubstantial variations of outputs. In a slight variation of thearrangement above described separate grid bias batteries are employedfor the various grid circuits instead of a common grid bias battery,these batteries being connected at their positive ends to suitablychosen tapping points upon that resistance, one end of which isconnected to earth and/or to the common cathode point.

Obviously the invention is applicable to almost any multiamplifierreceiver, the feature of the said invention being the successive controlfirst of one and then of increasing numbers of amplifying membersincorporated in the receiver. Obviously also in a frequency changingcircuit such as above described, it is not necessary that the variouscircuits should all be controlled in the particular order set forth. Forexample, as regards the frequency changing arrangements, it is possibleso to arrange'the control that both the amplitude of the incomingsignal, and that of the local heterodyne is reduced on the input gridcircuit of the modulating valve. Alternatively, as regards the frequencychanger, the control may be such that only the amplitude of theheterodyne is effected.

In a further modification shown in Fig. 3 the automatic gain controldevice is energized from the first detector or frequency changing valveFC instead of as shown in Fig. 1. The arrangement is the same as shownin Fig. 2 except for such obvious changes as are necessitated by thechanged position of the control unit AGC.

Fig. 4 shows a further modification in which the invention is applied tocontrol the frequency changer of a supersonic heterodyne receiverincorporating a frequency changer of the push-pull modulating type. Withsuch a frequency changer the power transferred to the intermediatefrequency amplifier may readily be made to be a direct function of theamplitude of the heterodyning wave simultaneously impressed upon thegrid circuits of the two modulating valves. In Fig. 4, which shows themodulating valves and associated ap aratus only, the local 0scillatorvalve LOWis coupled through a coupling valve CV to the mid-point of thecommon grid inductance for the modulating valves VM and -VM Thefrequency changer stage comprising these valves, of course is followedby the intermediate frequency amplifier. The grid bias of the couplingvalve CV is controlled through lead 3 in accordance with the output ofthe gain control rectifier V (see Fig. 2). The oscillatory circuit II inthe output circuit of the valve CV is tuned to a harmonic of thefrequency of the oscillator valve LOV 1. e., to a harmonic of thenatural frequency of the oscillatory circuit I. It will be seen that inthe arrangement shown in Fig. 4 the amplitude of the heterodyne waveapplied to the valves VM V'M is controlled by the device AGC.

It is possible to obtain control of the frequency changer in a mannerwhich results in control both of the signal and heterodyne waves byconnecting the lead 2 (of Fig. 2) as indicated in broken lines in Fig.4, i. e., merely by connecting the center point of the common gridcircuit of the Valves VM VM to the biasing battery E (Fig. 2) instead ofemploying the coupling valve bias control arrangement shown in fulllines in Fig. 4. The latter arrangement (that shown in full lines inFig. 4) which results in control of the heterodyne wave only, ispreferred although it is, of course, somewhat more expensive toconstruct.

While we have indicated and described several systems for carrying ourinvention into effect, it will be apparent to one skilled in the artthat our invention is by no means llmited to the particularorganizations shown and described, but that many modifications may bemade without departing from the scope of our invention as set forth inthe appended claims.

What we claim is:

1. An automatic ain control arrangement for a superhetero yne receiverwhich includes a radio frequency amplifier, first detector, intermediatefrequency amplifier and a second detector, said arrangement comprising adiode control tube having a cold electrode connected to an inputelectrode of said second detector for the impression of signal currentson said diode electrode, a control bias source connected in the circuitof said diode cold electrode, and a plurality of independent directcurrent connections from a plurality of points of increasing negativepotential on said source to the gain control electrodes of said seconddetector, intermediate frequency amplifier and said radio frequencyamplifier respectively, said points being so chosen that as the receivedsignals vary from weak to strong the biases of said gain controlelectrodes are successively altered in the aforesaid order.

2. An automatic gain control arrangement for a superheterodyne receiverwhich includes a radio frequency amplifier, first detector, intermediatefrequency amplifier and a second detector, said arrangement comprising adiode control tube having a cold electrode connected to an inputelectrode of said second detector for the impression of signal currentson said diode electrode, a control bias source connected in the circuitof said diode cold electrode, and a plurality of independent directcurrent connections from a plurality of points of increasing negativepotential on said source to the gain control electrodes of said seconddetector, intermediate frequency amplifier, first detector, and saidradio frequency amplifierv respectively, said points being so chosenthat as the received signals vary from weak to strong the biases of saidgain control electrodes are successively altered in the aforesaid order.

GASTON ADELIN MATHIEU. GERALD ARTHUR ISTED.

